The present invention relates to the in situ and ex situ oxidation of organic compounds in soils, groundwater and process water and wastewater and especially relates to the in situ oxidation of volatile organic compounds in soil and groundwater.
The presence of volatile organic compounds (VOCs) in subsurface soils and groundwater is a well-documented and extensive problem in industrialized and industrializing countries. As used in this specification and its appended claims, volatile organic compounds or VOCs means any at least slightly water soluble chemical compound of carbon, with a Henry""s Law Constant greater than 107 atm m3/mole, which is toxic or carcinogenic, is capable of moving through the soil under the influence of gravity and serving as a source of water contamination by dissolution into water passing through the contaminated soil due to its solubility, including, but not limited to, chlorinated solvents (such as trichloroethylene (TCE), vinyl chloride, tetrachloroethylene (PCE), dichloroethanes, ), benzene,, chlorobenzenes, ethylene dibromide, methyl tertiary butyl ether, halobenzenes, polychlorinated biphenyls, chlorophenols, acetone, ter-butyl alcohol, tert-butyl formate, anilines, nitrobenzenes, toluene, xylenes.
In many cases discharge of volatile organic compounds into the soil have lead to contamination of aquifers resulting in potential public health impacts and degradation of groundwater resources for future use. Treatment and remediation of soils contaminated with volatile organic compounds have been expensive and in many cases incomplete or unsuccessful. Treatment and remediation of volatile organic compounds that are either partially or completely immiscible with water (i.e., Non Aqueous Phase Liquids or NAPLs) have been particularly difficult. This is particularly true if these compounds are not significantly naturally degraded, either chemically or biologically, in soil environments. NAPLs present in the subsurface can be toxic to humans and other organisms and can slowly release dissolved aqueous or gas phase volatile organic compounds to the groundwater resulting in long-term (i.e., decades or longer) sources of chemical contamination of the subsurface. In many cases subsurface groundwater contaminant plumes may extend hundreds to thousands of feet from the source of the chemicals resulting in extensive contamination of the subsurface. These chemicals may then be transported into drinking water sources, lakes, rivers, and even basements of homes.
The U.S. Environmental Protection Agency (USEPA) has established maximum concentration limits for various hazardous compounds. Very low and stringent drinking water limits have been placed on many halogenated organic compounds. For example, the maximum concentration limits for solvents such as trichloroethylene, tetrachloroethylene, and carbon tetrachloride have been established at 5 xcexcg/L, while the maximum concentration limits for chlorobenzenes, polychlorinated biphenyls (PCBs), and ethylene dibromide have been established by the USEPA at 100 xcexcg/L, 0.5 xcexcg/L, and 0.05 xcexcg/L, respectively. Meeting these cleanup criteria is difficult, time consuming, costly, and often virtually impossible using existing technologies.
One technology, which has been attempted at pilot-scale test applications, is the use of potassium permanganate (KMnO4) alone as an oxidant for in situ soil remediation. (Treatment performed in situ does not involve physical removal of the contaminated phase itself, whereas, ex situ treatment methods involve physical removal of the contaminated phase and treatment elsewhere.) This has been attempted in view of KMnO4""s known capacity to oxidize target VOCs present at typical sites (e.g. trichloroethylene, dichloroethylene, and vinyl chloride). An example of such a reaction is: 2MnO4xe2x88x92+C2HCl32CO2+2MnO2+3Clxe2x88x92+H+
It is also well known that KMnO4 has versatile chemistry and high aqueous solubility. Once dissolved into aqueous phase, permanganate salts (such as potassium permanganate, sodium permanganate, calcium permanganate and the like) dissociate to form permanganate ions (MnO4xe2x88x92) that may transform to a variety of species with oxidation states of manganese in +1, +2, +3, +4, +5, +6, and +7. The most common species of manganese are manganese ions (Mn++), manganese dioxide (MnO2), and permanganate (MnO4xe2x88x92). The oxidation strength of (MnO4xe2x88x92) depends on the electron accepting capability of (MnO4xe2x88x92) which is pH dependent. The lower the pH, the greater the tendency of (MnO4xe2x88x92) to accept the electrons as indicated by the redox potential (E0) values in Eqs. 1 through 4:
The reactivity of KMnO4 depends on the reaction conditions and the types of organic compounds being oxidized.
While, chemically, potassium permanganate is effective at oxidizing unsaturated volatile organic compounds, currently known methods to use that ability to actually clean up a site require exceedingly large quantities of KMnO4 to overcome the natural oxidant demand exerted by the soil, thereby limiting, for a given amount of KMnO4, the percentage of KMnO4 available for oxidizing the volatile organic compounds. Large amounts of KMnO4 are thus required per unit of soil volume limiting the application of this technology due to high cost.
Another disadvantage of potassium permanganate, which has not been overcome by prior art clean-up methods, is the formation of solid manganese dioxide (MnO2) precipitates. This precipitate may result in clogging of the soil, resulting in a reduced permeability of the soil to water, reducing the hydraulic conductivity thereof, and thereby inhibiting oxidant access to the entire contaminated site rendering treatment of the soil and volatile organic compounds incomplete.
A further disadvantage of adding potassium permanganate alone and in large quantities for subsurface remediation is that it can result in the formation of soluble manganese compounds in groundwater that may exceed drinking water standards. For this and the foregoing reasons, attempts to date to use potassium permanganate for in situ applications have not been fully successful.
Early use of peroxydisulfate is reported for the purpose of organic compound synthesis. Additionally, thermally catalyzed decomposition of ammonium persulfate as a method of organic carbon digestion has been reported being accomplished at very low pH (i.e. in the vicinity of pH 2.0), but has not been thought to be useful for that purpose at higher pH. More recent publications have indicated that, under ambient temperature and uncatalyzed conditions, atrazine and PCBs may be oxidized by ammonium persulfate in aqueous solutions and in contaminant spiked soils under batch conditions. There has been no suggestion that this oxidation reaction has any application to the treatment of volatile organic compounds in contaminated soil or groundwater.
Divalent and heavy metal cation adsorption onto manganese oxide surfaces is a known phenomenon. The order of preference for selected cations to adsorb onto MnO2 surfaces is reported as follows:
Pb++ greater than Cu++ greater than Mn++ greater than Co++ greater than Zn++ greater than Ni++ greater than Ba++ greater than Sr++ greater than Ca++ greater than Mg++.
Stoichiometry and rates of redox interactions with manganese dioxide and various organic compounds in aqueous solutions have been studied for some organic compounds, such as aniline and primary aromatic amines; hydroquinone; various organic acids, substituted phenols, and chlorophenols. In all of the above systems reduction of the manganese dioxide to Mn++ results in the redox couple with the organic compound being oxidized, the reaction being identified in the literature as interfacial. There has been no recognition, however, that this knowledge has application to the removal of contaminants from soil.
The present invention relates to a method for the treatment of contaminated soil, sediment, clay, rock, and the like (hereinafter inclusively referred to as xe2x80x9csoilxe2x80x9d) containing volatile organic compounds, as well as for treatment of contaminated groundwater or wastewater containing volatile organic compounds.
The method of the present invention uses one or more water soluble oxidants under conditions which enable oxidation of most, and preferably substantially all, the volatile organic compounds in the soil, groundwater, and/or wastewater, rendering them harmless.
The oxidant may be solid phase water soluble peroxygen compound and/or a permanganate compound, introduced into the soil in amounts, under conditions and in a manner which assures that the oxidizing compound(s) are able to both 1) satisfy most and preferably substantially all the soil oxidant demand, and 2) contact and oxidize most, and preferably substantially all, the target VOCs, rendering the target VOCs harmless. As known and used herein, the term xe2x80x9csolid phasexe2x80x9d refers to the state of a compound in its pure phase at ambient temperatures and pressures. The soil oxidant demand referred to above can be created by various species including natural organic matter, reduced inorganic species such as ferrous ion, ferrous carbonate, and other allochthonous (anthropogenic) organic and reduced inorganic species.
In a preferred embodiment of the invention a peroxygen compound is introduced into the soil in sufficient quantities to satisfy the soil oxidant demand, and a permanganate compound is introduced into the soil in sufficient quantities to oxidize the VOCs and render them harmless. These compounds may be introduced or injected into the soil simultaneously, such as in a mixture, or sequentially. Since the permanganate compound will not have to satisfy the soil oxidant demand to any significant extent, the formation of undesirable amounts of soil clogging MnO2 precipitate, as occurred with prior art methods, is avoided, and the permanganate compound is readily able to reach and react with the target VOCs. This methodology may also be used ex situ to treat quantities of contaminated soil which have been removed from the ground. As used herein and in the appended claims, xe2x80x9csequentialxe2x80x9d introduction of the peroxygen compound and the permanganate compound is intended to mean introduction or injection of the compounds xe2x80x9cone after the otherxe2x80x9d (i.e. xe2x80x9calternatelyxe2x80x9d), and includes repeating the sequence as many times as necessary to achieve a desired result.
In another embodiment of the present invention, wherein only relatively low levels of VOCs and other organic compounds need to be treated, such as at a distant end of a groundwater plume extending downstream from a contaminated site which has been treated to remove a high percentage of the VOCs and other organic compounds, a permanganate compound alone is introduced into the ground in the path of the contaminated groundwater plume. The permanganate compound creates a zone of material through which the groundwater passes and within which the VOCs and other organic compounds in the groundwater are oxidized. The permanganate compound, when introduced into the soil, will initially react with constituents in the soil to form a xe2x80x9cbarrierxe2x80x9d zone of MnO2 precipitate. The VOCs and other organic compounds in the groundwater readily attach themselves to the MnO2 by adsorption. Reduction of the manganese and oxidation of the VOC then takes place within the zone, resulting in the elimination of the VOCs.
According to another aspect of the present invention, under conditions where metal cations are present in the contaminated soil, persulfate may be introduced into the contaminated soil to remove VOCs. The metal cations catalytically decompose the persulfate to form sulfate free radicals, which oxidize the target VOCs. If the metal cations are not naturally present in sufficient quantities, they may be added from an external source.
The foregoing and other features and advantages of the present invention will become clear from the following description.
In accordance with an exemplary embodiment of the present invention, the oxidation of volatile organic compounds at a contaminated site is accomplished by the sequential injection of persulfate and then permanganate into the soil.
Alternating injection of the persulfate and permanganate entails introducing sufficient persulfate into the soil to satisfy a sufficient amount of the soil oxidant demand such that, upon the introduction of the permanganate, the permanganate does not excessively react with the normal soil constituents, as it would if used alone. By xe2x80x9cexcessivelyxe2x80x9d it is meant enough to form MnO2 precipitate in quantities that reduce the soil permeability and diffusivity to the point where the permanganate cannot readily move through the soil to reach and oxidize the VOCs. Due to the lessening of the soil oxidant demand as a result of the persulfate, a faster and more uniform distribution of the permanganate through the soil to the target contaminant is enabled and much less permanganate is required to oxidize the VOCs. However, as the amount of volatile organic compounds decrease, to react with the remaining volatile organic compound the permanganate will need to migrate through additional soil that has an additional soil oxidant demand. This may require an additional injection of persulfate at that location. This sequential injection of persulfate and permanganate would be repeated, as and if required, to oxidize VOCs within the soil volume being treated until the VOC concentration is reduced to the desired level.
In a preferred form of the invention, sodium persulfate (Na2S2O8) is introduced into the soil, followed by potassium permanganate (KMnO4). The persulfate satisfies the oxidant demand of the soil by oxidizing the soil constituents, resulting in less of those constituents being available to react with the permanganate. (The persulfate reaction is relatively slow; and it may be desirable, although it is not required, to wait long enough for the persulfate reaction to go to completion before starting the permanganate.) Because less permanganate reacts with the soil, more is available to oxidize the VOCs in the soil. Further, the reduction of (MnO4xe2x88x92) to the solid precipitate MnO2 is lessened. Thus, there is less precipitate to reduce permeability of the soil and restrict the potassium permanganate from reaching, reacting with and destroying VOCs. In other words, the introduction of both the sodium persulfate and the potassium permanganate into the soil, allows the potassium permanganate to more quickly and more uniformly move through the soil to the target VOCs, rather than forming an unacceptable amount of cementitious-like solid precipitate. (This process may be initiated by the use of injection means, such as wells for in situ application, or by nozzles, pipes or other conduits to inject the oxidants into soil which has been removed from the ground for ex situ treatment.)
For in situ soil treatment, injection rates must be chosen based upon the hydrogeologic conditions, that is, the ability of the oxidizing solution to displace, mix and disperse with existing groundwater and move through the soil. Additionally, injection rates must be sufficient to satisfy the soil oxidant demand and chemical oxidant demand in a realistic time frame. It is advantageous to clean up sites in both a cost effective and timely manner. Careful evaluation of site parameters is crucial. It is well known that soil permeability may change rapidly both as a function of depth and lateral dimension. Therefore, injection well locations are also site specific. Proper application of any remediation technology depends upon knowledge of the subsurface conditions, both chemical and physical, and this process is not different in that respect.
For ex-situ oxidation of VOCs, type of oxidants and their application rates must be determined based on oxidant demand of the contaminated soil or water. Selection of operating conditions (such as extent of mixing, temperature, quantities, and rates of the selected oxidants, duration of treatment and sequencing of oxidant application) are made based on the oxidant demand exerted by the contaminated soil/water and quantity and type of target VOCs to be treated.
Ex-situ treatment systems may include, but are not limited to: (1) above ground tanks, rail cars, trucks, tanks or other commercial transport vessels with or without mixing systems; (2) storage drums; and (3) pipes and other water, soil and soil slurry transfer and conveyance system.
While potassium permanganate is preferred, in view of its lower cost, any compound that dissociates into the desired permanganate ion (MnO4xe2x88x92) will work. Examples of other possible permanganates useful in the method of the present invention are sodium permanganate and calcium permanganate, in order of increasing cost. At an ambient temperature, the aqueous solubility of KMnO4 is about 60 g/L, while that of NaMnO4 is about 600 g/L. Upon dissolution in water, both dissociate to generate (MnO4)xe2x88x92 ions that undergo various reactions. Although a primary issue is often cost, NaMnO4, due to its order of magnitude greater solubility relative to KMnO4, could be useful whenever the soil permeability is very low and only a small amount of liquid can travel from the injection point toward the contaminant. Additionally, potassium ions have been shown to cause the swelling of certain clays that could lead to permeability reductions. The use of sodium ions, in selected instances, could eliminate such difficulties.
Similarly, while sodium persulfate is the preferred compound for oxidizing the soil constituents, other solid phase water soluble peroxygen compounds comprising a bivalent oxygen group, Oxe2x80x94O, can be employed. Such compounds include all the persulfates, and the like, with the persulfates being preferred because they are inexpensive and survive for long periods in the groundwater saturated soil under typical site conditions. The persulfate anion is the most powerful oxidant of the peroxygen family of compounds. Although the persulfate ion is a strong two-electron oxidizing agent with a standard reduction potential of 2.12 v, in the majority of its reactions persulfate undergoes either a one-electron reduction with formation of one sulfate radical ion (and hence has effectively lower reduction potential than 2.12 v) or a breakage of the weak oxygen-oxygen bond with formation of two sulfate radical-ions. The former reaction is represented by the following equation:
The second reaction generally occurs when solutions of persulfates are sufficiently heated, and is represented by the following equation:
xe2x80x83S2O8xe2x88x92xe2x88x92+Heat2SO4xe2x88x92
Similarly, free radicals can also be generated in the presence of transition metal ions, such as Fe++, as follows:
S2O8xe2x88x92xe2x88x92+Fe++Fe++++SO4xe2x88x92xe2x88x92+SO4xe2x88x92
The highly reactive sulfate radical-ion may undergo reactions with a variety of substrates present in the solution. In addition, the one-electron oxidation intermediate of the substrate may be a reactive intermediate, which may further react with other substrates present in the solution or the peroxide ion. Thus, depending on the reaction conditions and type of substrate present, persulfate may follow a direct oxidation pathway, radical formation, or both.
The most preferred persulfate is sodium persulfate as it has the greatest solubility in water and is least expensive. Moreover, it generates sodium and sulfate upon reduction, both of which are relatively benign from environmental and health perspectives. Potassium persulfate and ammonium persulfate are examples of other persulfates which might be used. Potassium persulfate, however, is an order of magnitude less soluble in water than sodium persulfate; and ammonium persulfate is even less desirable as it may decompose into constituents which are potential health concerns.
The following are other examples of reactions of KMnO4, MnO4xe2x88x92, and S2O8xe2x88x92xe2x88x92with selected organic and inorganic species:
S2O8xe2x88x92xe2x88x92+2Fe++2Fe++++2SO4xe2x88x92xe2x88x92
S2O8xe2x88x92xe2x88x92+NO2xe2x88x92NO3+2SO4xe2x88x92xe2x88x92+2H+
S2O8xe2x88x92xe2x88x92+HCOOxe2x88x92CO2+HSO4xe2x88x92+SO4xe2x88x92xe2x88x92
S2O8xe2x88x92xe2x88x92+2Cr(III)2Cr(VI)+2SO4xe2x88x92xe2x88x92
3C6H5OH+28KMnO4+5H2O18CO2+28KOH+28MnO2
xe2x80x832MnO4xe2x88x92+3Mn+++2H2O5MnO2+4 H+
8MnO4xe2x88x92+3Sxe2x88x92xe2x88x92+4H2O5MnO2+3SO4xe2x88x92xe2x88x92+8OHxe2x88x92
For reasons of economy and purity of the peroxygen compound, it may be advantageous to produce the peroxygen on site near the point of use.